In the information age, the importance of keeping data on-line at all times is becoming paramount, even as the amount of stored information is growingly exponentially. The need for Business Continuance (BC) and fast data recovery is acute and well-acknowledged. Today's businesses are extremely vulnerable to data loss and system downtime. The cost of data loss or an hour of downtime can result in losses up to millions of dollars. The most commonly adopted techniques to protect and recover data include Point-in-time data copy (PIT, also known as snapshot), backup and restore, remote data replication, or any combination of these. Each of these techniques is designed to deal with specific types of data loss or system failure scenarios.
PIT is designed to allow recovery of data from a copy at a time in the past when the snapshot was taken. The changes that were made after the last snapshot are usually lost. Typically, data will be inaccessible to applications while the recovery takes place. However, this technique usually cannot help in cases of system failure. Similarly, the backup and restore technique cannot guarantee zero data loss, since the changes that were made after the last backup are typically lost. If the backup was stored on tapes the recovery could be very slow and the system could be down for an unacceptable length of time. Remote data replication is another technique and comes in two flavors, synchronous and asynchronous. Only synchronous remote mirroring can avoid data loss during site-wide disasters, since a write from a calling application is not considered complete until the data is written successfully to both the local site and the remote site. However, this has a performance penalty on the applications. In asynchronous remote mirroring a write is considered to be completed just after writing to the local site. Subsequently, the updates are sent to the remote site as well. Thus, in a site-wide disaster, there would be data lost if there was some data pending to be sent to the remote site. With remote mirroring in place, it is possible to guarantee continuous data access in the presence of site wide failures by providing hot stand-by hosts and applications at the remote site and directing clients to the remote site when the primary site encounters a failure. In practice, such a solution is cost prohibitive for Small and Medium Businesses (SMBs) due to its purchase and maintenance costs.
Therefore, there is a need for an affordable solution which can provide a low probability of failure, and is capable of minimizing system downtime in response to a system failure. Such a solution would be beneficial for a wide range of systems where data is vulnerable to loss or unavailability